Effect of a phosphoric acid treatment on the high temperature oxidation behaviour of γ-TiAl: An overall mechanism

TiAl coupons were dipped in a low-concentration phosphoric acid solution, air dried, then heated up to 700 °C or 800 °C before discontinuous oxidation under laboratory air at these temperatures. At the end of the heating ramp, surfaces of TiAl samples were covered by a layer made of a pyrophosphate compound, resulting from reaction between the deposited acid and oxidized titanium. This layer strongly adhered to the substrate surface and allowed to greatly improve the resistance of TiAl to oxidation. After a certain time, a transition period occurred during which pyrophosphate compound disappeared to be changed into TiO₂ rutile. For experiments carried on at 800 °C, it was shown that the phosphorus coming from the pyrophosphate group was present in this titanium oxide and that sample mass gain was still lowered. Performed analyses (XRD, SEM, EDS, μ-Raman spectroscopy) allowed to propose an overall mechanism to account for the “phosphorus effect”.     

Effect of a sputtered TiAlCr coating on the oxidation resistance of TiAl intermetallic compound

The effect of a sputtered TiAlCr coating on the oxidation resistance of TiAl intermetallic compound was investigated in static air. The bare TiAl alloy exhibited poor isothermal and cyclic-oxidation resistance at 800–1000°C due to the formation of TiO₂-base scales which tend to spall during cooling. A sputtered Ti-50Al-10Cr coating remarkably improved the oxidation resistance of TiAl, due to the formation of an adherent Al₂O₃ scale at 800–1000°C. After long-term oxidation (at 900°C for 1000 hr), TiAlCr coating still provided excellent protection for the TiAl alloy. Minor interdiffusion occurred due to the inward diffusion of Cr, while no Kirkendall voids were found at the coating/ substrate interface. In contrast, NiCrAlY and CoCrAlY coatings reacted extensively with the TiAl alloys. Moreover, the TiAlCr coating alloy is based on -TiAl and TiAlCr Laves phases, which may offer improved mechanical properties. The TiAlCr coating exhibited a better combination of oxidation resistance and substrate compatibility than conventional aluminide and MCrAlY coatings.     

The oxidation resistance of a sputtered,microcrystalline TiAl-intermetallic-compound film

The oxidation behavior of a cast TiAl intermetallic compound and its sputtered microcrystalline film was investigated at 700–900°C in static air. At 700°C, both the cast alloy and its sputtered microcrystalline film exhibited excellent oxidation resistance. No scale spallation was observed. However, at 800–900°C, the oxidation kinetics for the cast TiAl alloy followed approximately a linear rate law, which indicates that it has poor oxidation resistance over this temperature range. The poor oxidation resistance of TiAl was due to the formation of an Al₂O₃+TiO₂ scale which spalled extensively during cooling. Nevertheless, the sputtered, TiAl-microcrystalline film exhibited very good oxidation resistance. The oxidation kinetics followed approximately the parabolic rate law at all temperatures. Although the composition of the scales was the same as that of scales formed on the cast alloy, the scales formed on the sputtered microcrystalline-TiAl film are adherent strongly to the substrate. No scale spallation was found at 700–850°C, while a small amount of spallation was observed only at 900°C. This indicates that microcrystallization can improve the oxidation resistance of the TiAl alloy.     

Effect of chromium on the oxidation resistance of TiAl intermetallics

The effect of 10 at.%Cr on the oxidation resistance of TiAl intermetallic compound at 800–1100°C in air was investigated. The results indicated that 10 at.%Cr equally substituting for Ti and Al in TiAl alloy had duplex effects on the isothermal kinetics of DAL At lower temperatures (800–900°C), Cr increased the oxidation rates as a result of the doping effect of Cr in the scale and at higher temperatures (1000–1100°C), especially at 1100°C, Cr significantly reduced the oxidation rates as a result of the formation of a continuous Al₂O₃ film on the surface. 10 at.%Cr only substituting for Ti in TiAl alloy remarkably reduced the oxidation rates at all temperatures by about two orders of magnitude. Moreover, 10 at%Cr significantly improved the cyclic-oxidation rsistance of TiAl alloy.     

Isothermal oxidation behavior of powder metallurgy beta gamma TiAl–2Nb–2Mo alloy

A new TiAl–2Nb–2Mo beta gamma alloy was synthesized by powder metallurgy process. HIP’ed and vacuum heat treated specimens were isothermally oxidized at 800 °C and 900 °C in air up to 500 h. The TiAl–2Nb–2Mo alloy oxidized parabolically up to 500 h at both 800 °C and 900 °C. The oxides consisted of outer TiO₂ layer, intermediate Al₂O₃ layer, and inner TiO₂ rich mixed layer and the oxidation mechanisms of the alloy were identical at both temperatures. During oxidation, the degradation of lamellar colonies formed a diffusion zone just below the oxide/substrate interface consisting of γ-TiAl matrix and dispersed beta phases which contained high concentration of Nb and Mo. The oxidation rate of the TiAl–2Nb–2Mo alloy is more sensitive to temperature than those of the Ti–48Al–2Nb–2Cr and Ti–48Al–2Nb–2Cr–W alloys.     

Improving the high-temperature oxidation resistance of a β-γ TiAl alloy by a Cr2AlC coating

A Cr₂AlC coating was deposited on a β-γ TiAl alloy. Isothermal oxidation tests at 700 °C and 800 °C, and thermocyclic oxidation at 800 °C were performed in air. The results indicated that serious oxidation occurred on the bare alloy. Thick non-protective oxide scales consisting of mixed TiO₂ + α-Al₂O₃ layers formed on the alloy surface. The coated specimens exhibited much better oxidation behaviour by forming an Al-rich oxide scale on the coating surface during the initial stages of oxidation. This scale acts as diffusion barrier by effectively blocking the ingress of oxygen, and effectively protects the coated alloys from further oxidation.     

Oxidation protection of gamma-titanium aluminide using glass-ceramic coatings

γ-TiAl intermetallic alloys are presently considered an efficient structural material for advanced turbine blades and aero-engine components due to their various advantages compared to the traditionally used superalloys. However, their poor oxidation resistance at temperatures > 750 °C severely limits their wider application. The present study dealt with the improvement of oxidation resistance of this alloy by applying impervious glass-ceramic coatings by vitreous enameling technique. Results showed that MgO-SiO₂-TiO₂ glass-ceramic coating could offer excellent oxidation resistance to γ-TiAl at 800 °C even up to 100 h with negligible weight gain (~ 0.10 mg/cm²) compared to that of the bare alloy (~ 1.3 mg/cm²). The coatings those were belonging from BaO-MgO-SiO₂, ZnO-Al₂O₃-SiO₂ and BaO-SiO₂ systems also extend appreciable improvement in the oxidation resistance of the alloy at 800 °C up to 100 h. At further higher temperature such as at 1000 °C, the ABK-13 and ABK-103 glass-ceramic coatings offered significant protection to the alloy up to 25 h of exposure in air with minimum weight gain (~ 0.34 mg/cm²). However, after that the coated layers started to peel off from the alloy surface.     

Air-oxidation of nano-multilayered CrAlSiN thin films between 800 and 1000 °C

Multilayered CrAlSiN films consisting of crystalline CrN nanolayers and amorphous AlSiN nanolayers were deposited by the cathodic arc plasma deposition. The oxidation characteristics of the films were studied at temperature range from 800 and 1000 °C for up to 100 h in air. During their oxidation, the amorphous AlSiN nanolayers crystallized. The films displayed good oxidation resistance, owing to the formation of oxide crystallites of Cr₂O₃, α-Al₂O₃, and amorphous SiO₂. The oxidation of the CrAlSiN films occurred via complex routes such as the outward diffusion of Cr, Al and nitrogen and inward transport of oxygen.     

Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders

The oxidation behavior of hot-pressed Al₂O₃–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al₂O₃–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al₂O₃–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.     

TEM investigations of the early stages of TiAl oxidation

The early stages of TiAl oxidation at 900°C and 1000°C in air have been investigated by transmission electron microscopy (TEM). The investigations revealed that at the beginning of oxidation, i.e., after 4 min, TiO₂ and Al₂O₃ grow in a preferential orientation on the -TiAl substrate. After 4 h of oxidation an oxide scale structure can already be found similar to that known from long-term oxidation. In addition, besides -Al₂O₃, the formation of a second aluminum oxide phase and of titanium nitrides is observed. The processes at the metal-oxide interface of oxidation in the early stages, consisting of a repeated cycle of Al₂O₃ formation, Al₂O₃ dissolution, outward migration of Al through the scale, and reprecipitation of Al₂O₃ in the outer scale, are described by a model. The four stages observed in the kinetics of TiAl oxidation are explained on the basis of the results obtained and the structure of the oxide scale.     

Oxidation behavior of magnetron sputtered double layer coatings containing molybdenum,silicon and boron

Protective coating systems were applied to Mo–9Si–8B (at.%) alloys to prevent oxidation at elevated temperatures. The coatings produced by magnetron sputtering and subsequent annealing consisted of an outer oxidation protection layer and an interlayer between this and the substrate. Three amorphous outer layers with different compositions were deposited: Mo–45Si–25B, Mo–55Si–10B and Mo–29Si–15B (all in at.%). The interlayer was selected to give a diffusion barrier with the composition of the Mo₅SiB₂ (T2) phase. All coatings were dense and well-adherent. During vacuum annealing the amorphous as-deposited coatings became crystalline exhibiting mainly the intermetallic Mo₅SiB₂ compound as interlayer and the MoSi₂, Mo₅Si₃ and MoB phases in the top layers. The samples were exposed to dry laboratory air in the pesting regime at 800 °C and above, i.e. at 1000 and 1300 °C for up to 100 h under cyclic conditions. All coatings were protective at 800 and 1000 °C for at least 100 h and showed a marked improvement in mass change compared to the uncoated substrate. For protection at 800 °C higher boron content is preferential, while at higher oxidation temperatures a lower boron content provides improved oxidation protection. At 1300 °C stress induced failures like cracking, spallation and buckling occurred due to the relatively high CTE mismatch between PVD coating and substrate. Even though, the mass change was still markedly reduced as compared to the bare substrate.     

Hot corrosion behaviour of Yb2Zr2O7 ceramic coated with V2O5 at temperatures of 600-800 °C in air

To better understand the hot corrosion behaviour of Yb₂Zr₂O₇ ceramic in molten V₂O₅, hot corrosion experiments were performed in a temperature range of 600-800 °C in air. Different reaction products of ZrV₂O₇, YbVO₄ and m-ZrO₂ were identified depending upon the hot corrosion conditions, for example, ZrV₂O₇ and YbVO₄ at 600 °C for 2 h and 8 h; ZrV₂O₇, m-ZrO₂ and YbVO₄ at 700 °C for 2 h; m-ZrO₂ and YbVO₄ either at 800 °C for 2 h or at 700-800 °C for 8 h. The hot corrosion reaction mechanisms were further discussed based on the thermal instability of ZrV₂O₇ at elevated temperatures.     

Oxidation resistance of TiAl3-Al composite coating on orthorhombic Ti2AlNb based alloy

The TiAl₃-Al composite coating on orthorhombic Ti₂AlNb based alloy was prepared by cold spray. Oxidation in air at 950 °C indicated that the bare alloy exhibited poor oxidation resistance due to the formation of TiO₂/AlNbO₄ mixture and intended to scale off at the TiO₂ rich zone. A nitride layer about 2 µm was formed under the oxide layer. The oxygen invaded deeply into the alloy and caused severe microhardness enhancement in the near surface region. The TiAl₃-Al composite coating exhibited parabolic oxidation kinetics and showed no sign of degradation after oxidized up to 1098 h at 950 °C in air under quasi-isothermal condition. No scaling of the coating was observed after oxidized at 950 °C up to the tested 150 cycles. The major oxide in the oxidized coating was Al₂O₃. The AlTi₂N, TiAl and small amount of TiO₂ were also observed in the oxidized coating. The EPMA and microhardness tests showed that inward oxygen diffusion was prevented by the interlayer, which was formed between the composite coating and the substrate during heat-treatment. Microstructure analyses demonstrated that the interlayer play a major role in protecting the substrate alloy from high temperature oxidation and interstitial embrittlement.     

High-Temperature Oxidation of Ti0.3Al0.2N0.5 Thin Films Deposited on a Steel Substrate by Ion Plating

To improve the high-temperature oxidation resistance of STD61 steels used ashot dies or cutting tools, Ti_0.3Al_0.2N_0.5films were deposited on STD61 steel substrates by arc-ion plating. Thedeposited film consisted of Ti₃Al₂N₂ andTi₂N phases. The oxidation characteristics were studied attemperatures ranging from 700 to 900°C in air. The deposited STD61steels displayed excellent oxidation resistance up to 800°C, butexhibited large weight gains and breakaway oxidation at 900°C. Theoxidation products were primarily Fe₂O₃, TiO, TiO₂,and -Al₂O₃, the relative amount of each oxidebeing dependent on the oxidation condition. Among various oxides, TiO₂and -Al₂O₃ were the major oxides at 800°Cfor at least up to 16 hr. However, at a higher temperature or a longeroxidation period, the significant outward diffusion of iron from thesubstrate resulted in the formation of iron oxides, together with otheroxides of Ti and Al.     

Hydrothermal corrosion of TiN PVD films on SUS-304

Hydrothermal corrosion of thin TiN PVD films (3 μm thickness) at 100 MPa water and 20-800 °C temperature range was studied. Noticeable oxidation starts above 200 °C and acceleration of oxidation processes takes place in hydrothermal conditions in comparison with airflow oxidation of corresponding PVD films and air oxidation of TiN powders. The formation of Ti_nO_2n−1 homological series phases and regular single crystals in oxide scale were observed. FeTiO₃ ilmenite layer secures high protective properties at 600 °C and practically does not contain chromium. Therefore, usual low alloy steel with TiN coating can be used instead superalloy for wet air oxidation system with working temperature up to 600 °C.     

Oxidation behaviour of Al2O3–TiC–Co composites at 800–1000 °C in air

The oxidation behaviour of nanometre and micrometre sized Al₂O₃–TiC–Co composites is investigated at 800–1000 °C in air for 25 h. The oxidation resistance of nanometre sized samples is better than of micrometre sized. Phase compositions and microstructures were studied by XRD and SEM. The values of general rate constant k and oxidation exponent n are dependent on oxidation temperature and composites. The oxidation kinetics followed a rate that is slightly faster than the parabolic-rate law at 800–1000 °C. The activation energy of the nanometre sized is higher than of micrometre sized in the range of 800–1000 °C.     

Oxidation behaviour of Ti-46.7Al-1.9W-0.5Si in air and Ar-20%O2 between 750 and 950 °C

The oxidation behaviour of an intermetallic alloy, Ti-46.7Al-1.9W-0.5Si, was studied in air and Ar-20%O₂ atmospheres at 750, 850 and 950 °C. Oxidation of the alloy followed a parabolic rate law at low temperature (750 °C) in both environments. The alloy oxidised parabolically in air and at a slower rate in Ar-20%O₂ at 850 °C. Following a parabolic oxidation for a relatively short exposure period (72 h) at 950 °C, the oxidation rate was reduced after prolonged exposure (up to 240 h) in air. The alloy oxidised in a slower manner in the Ar-20%O₂ atmosphere at 950 °C. Higher oxidation rates were observed in air than in Ar-20%O₂ at all three experimental temperatures. Multi-layered scales developed in both environments. The scale formed in air consisted of TiO₂/Al₂O₃/TiO₂/TiN/TiAl₂ layers, ranging from the surface to the substrate—whilst the scale developed in the Ar-20%O₂ atmosphere comprised of the sequence TiO₂/Al₂O₃/TiO₂/Al₂O₃/Ti₃Al/substrate. The two layers of Al₂O₃ in Ar-20%O₂ were more effective in providing protection of the substrate against high temperature corrosion than the single layer of Al₂O₃ formed in air.     

High-temperature oxidation of rhodium

The oxidation kinetics of Rh were measured in air at 1 atm. in the temperature range 600–1000°C. The oxidation weight gain proceeds logarithmically at the lower temperatures (600°C, 650°C) followed by a transition to power law behavior at the higher temperatures (800°C). The logarithmic growth kinetics result from thickening of a hexagonal Rh₂O₃ scale. The transition from logarithmic to power law growth kinetics occurs in the range 700–800°C, and reflects thickening of hexagonal and orthorhombic Rh₂O₃ scales. Above 800°C, the growth kinetics result from thickening of a predominately orthorhombic Rh₂O₃ scale. At 1000°C the oxide becomes volatile, leaving the metal surface exposed.     

The Oxidation of Fe-2 and 5 at.% Y Alloys at 600-800°C in Air

The oxidation of Fe-Y alloys containing 2 and 5at.% Y and pure iron has been studied at 600-800°Cin air. The oxidation of pure iron follows the parabolicrate law at all temperatures. The oxidation of Fe-Y alloys at 600°C approximatelyfollows the parabolic rate law, but not at 700 and800°C, where the oxidation goes through severalstages with quite different rates. The oxide scales on Fe-2Y and Fe-5Y at 700 and 800°C arecomposed of external pure Fe oxides containingFe₂O₃,Fe₃O₄, and FeO, with FeO being themain oxide and an inner mixture of FeO andYFeO₃. The scales on Fe-2Y and Fe-5Y at 600°C consist ofFe₂O₃,Fe₃O₄, andY₂O₃, with a minor amount of FeO.Significant internal oxidation in both Fe-Y alloysoccurred at all temperatures. The Y-containing oxidesfollow the distribution of the original intermetalliccompound phase in the alloys. The effects of Y on theoxidation of pure Fe are discussed.     

Formation of crystalline Al-Ti-O thin films and their properties

The article reports on the effect of addition of Ti into Al₂O₃ films with Ti on their structure, mechanical properties and oxidation resistance. The main aim of the investigation was to prepare crystalline Al-Ti-O films at substrate temperatures T_s ≤ 500 °C. The films with three different compositions (41, 43 and 67 mol% Al₂O₃) were reactively sputtered from a composed Al/Ti target and their properties were characterized using X-ray diffraction (XRD), X-ray fluorescent spectroscopy (XRF), microhardness testing, and thermogravimetric analysis (TGA). It was found that (1) the addition of Ti stimulates crystallization of Al-Ti-O films at lower substrate temperatures, (2) Al-Ti-O films with a nanocrystalline cubic γ-Al₂O₃ structure, hardness of 25 GPa and zero oxidation in a flowing air up to ∼ 1050 °C can be prepared already at low substrate temperature of 200 °C, and (3) the crystallinity of Al-Ti-O films produced at a given temperature improves with the increasing amount of Ti. The last finding is in a good agreement with the binary phase diagram of the TiO₂-Al₂O₃ system.